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    Re: Star-star distances for arc error
    From: Douglas Denny
    Date: 2009 Jun 23, 09:57 -0000
    ----- Original Message -----
    Sent: Tuesday, June 23, 2009 2:12 AM
    Subject: [NavList 8761] Re: Star-star distances for arc error

    And you wrote:
    "Attempting to use star separations to try to determine scale accuracy for example would not be possible due to the variables in the measurements themselves - including the refraction component even if calculated."

    A sure indication that you have never tried it!
    I have tried it and I don't rate it as practical.
    If accuracy is required, the measurements are difficult to achieve in the first place unless the sextant is clamped. Clamping requires special arrangements for tilting the sextant.
    Also, combining of two stars overlapping, with the inevitable abberrations seen in the telscope image itself preclude this to being within one minute at best. 
    Also, the measurement can only be done to an accuracy of the divisions of the sextant which in most cases is one minute of arc. 
    Sextants vary a lot.   My Hughes sextant - beautifuly built, brass frame, with platinum divided scale,  with drum/worm drive has a vernier to measure to 10 seconds of arc but the certificate indicates accuracy of max error of 1 minute 30 seconds at points on the scale.
    My Carl Zeiss Frieberger sextant  however, does not have a vernier so only measures to one minute of arc (half a minute estimated as best accuracy),  but the certificate gives max error of the scale as 24 seconds of arc.
    To attempt to calibrate the scale error using star distances is simply not accurate enough in my opinion.
    Any measurement of that nature needs the accuracy confidence to be at least an order greater in accuracy than the measurement errors likely  - and you are suggesting it is OK to measure two stars where the refraction error alone can be greater than the measurement.
    Even when corrections are applied,  if the altitude is less than 40 degress the refraction component is one minute of arc, (i.e. of an order comparable with the measurement);  if less than twenty degrees it is 2.6 minutes of arc;  and less than ten degrees  it is over 5 minutes of arc and the unknown effects of barometric pressure and humidity affecting density make low altitudes very suspect indeed.
    No. I am sorry, but as a method for checking sextant scales it is just not good enough in my opinion. 
    The only proper way is to use a dividing table and collimator where the dividing head is at least accurate to one tenth of a minute of arc at all readings.
    It is easy to get carried away with the theory without considering the practical difficulties or potential errors of the measurement itself compared to what one is measuring.
    Douglas Denny.
    Chichester. England.

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